Add mention of custom growfactors file in docs

docs/usage/tcja_after_2025.md

@@ -32,6 +32,14 @@ expire).  Compatible input datasets include:
   data, and are available only to those with access to the 2015
   IRS/SOI PUF
 
+Note that if you are using the Python API to Tax-Calculator (as
+described in the
+[Recipes](https://taxcalc.pslmodels.org/recipes/index.html)) to
+analyze the TCJA after 2025 and need to specify custom GrowFactors
+after 2025, you can use the old additive-differences `growdiffs`
+approach or you can specify a custom `growfactors` file as described
+[here](https://github.com/PSLmodels/Tax-Calculator/pull/2757#issuecomment-2218662804).
+
 Before reading the rest of this document, be sure you understand how
 to use the Tax-Calculator command-line tool
 [tc](https://taxcalc.pslmodels.org/guide/cli.html), particularly the